In general, cerium oxide is a high functional ceramic material that is widely used in catalysts, phosphors, cosmetics and abrasives. Recently, cerium oxide has been spotlighted as an abrasive in a STI (Shallow Trench Isolation) process and as an abrasive for optical glass. Such cerium oxide is generally prepared by a liquid-phase process, a solid-phase process, or the like. In the liquid-phase process, a pH modifier is added to a trivalent or tetravalent cerium starting material to directly form cerium oxide powder. On the other hand, in the solid-phase process, cerium oxide powder is formed by providing an intermediate product such as cerium carbonate and by performing a firing step.
Although the former process is useful for preparing fine particles because particles grow from a nucleation step, it has difficulty in preparing particles with a high crystallinity. On the other hand, the latter process has a disadvantage in that it produces macroparticles, which, in turn, should be processed into powder with a desired size by using a pulverization step.
Meanwhile, in one embodiment of the methods for preparing cerium carbonate powder in a solution according to the prior art, 0.05M cerium nitrate anhydrate is subjected to a hydrothermal reaction with 0.05˜1.0M urea at a temperature of 160° C., while controlling size of the resultant powder to 200 nm˜10 μm (Hsien-Cheng Wang, Chung-Hsin Lu, Synthesis of cerium hydroxycarbonate powders via a hydrothermal technique, Material Research Bulletin 1844, 1-10 (2002)). However, the above method is not amenable to mass production.
Additionally, in another embodiment of the methods for preparing cerium carbonate powder according to the prior art, cerium chloride, cerium sulfide or cerium nitrate anhydrate is subjected to a hydrothermal reaction with urea, while controlling crystallinity of the resultant powder depending on the kind of the salt, reaction temperature and reaction time, and urea concentration (M. Hirano, E. Kato, Hydrothermal synthesis of two types of cerium carbonate particles, Journal of Material Science Letter, 18, 403-405 (1999)). However, the above method is dangerous because it should be carried out at high temperature under high pressure. Also, the above method is not cost-efficient due to the cost required for the reaction system.
Further, in still another embodiment of the method for preparing cerium carbonate powder according to the prior art, cerium nitrate anhydrate and urea are used in a low concentration and the starting materials are subjected to a microwave process at a temperature of 80° C. so as to control the resultant powder to have a uniform spherical shape (Yasuro Ikuma, Hideyuki Oosawa, Eriko Shimada, Michiyo, Kamiya, Effect of microwave radiation on the formation of Ce2O(CO3)2.H2O in aqueous solution, Solid State Ionics 151, 374-352 (2002)). However, the above method has difficulty in providing a mass production system and is not amenable to bulk production.